Composite films are known and have been used to make various articles such as magnetic recording tapes and discs, abrasives, flexible printing plates, printing label stock, adhesive tapes, photographic film, bags or pouches, uncoated films and the like.
Even though such films have many applications, there continues to be a desire and a need to provide better adhesion between the individual layers. A number of techniques have been employed for achieving such improved adhesion. They have included subjecting one or more of the surfaces to be joined to high energy (e.g., corona discharge, electron beam discharge, flame treatment and the like). Other techniques have included chemical treatment of one or more of such surfaces; and application of primers to one or more of the surfaces. Although these techniques have met with some success, they each require either that the surface of the film be treated before a top layer is applied, or that the entire film be subjected to UV light. This, of course, requires the use of additional equipment and processing time. Also, these techniques generally must be specifically designed to meet the requirements and/or characteristics of the specific end use. Furthermore, films produced by these techniques often have relatively low inter-layer bond strength (for example, from 400 to 1400 g/cm width).
Another technique to improve interlayer adhesion involves exposure of the joined surfaces to ultraviolet light such as is described in U.S. Pat. No. 3,188,266. Films made by this technique include polyester film bearing a polyolefin-based layer. These films, while finding wide utility as heat-sealable films, still exhibit inter-layer bond strengths only in the above-described range. Moreover the interlayer bond strengths of such films tend to diminish substantially with age.
Other techniques which generally address improving adhesion of one material to another are disclosed in U.S. Pat. No. 4,418,164, U.S. Pat. No. 4,749,617, and EPO 86.304526.6. U.S. Pat. No. 4,418,164 discloses coating aramid filaments with polyfunctional aziridines. The coated filaments are then totally encased in unsaturated polyester matrices to reinforce the polyester. The reinforced polyester is used as a casting resin to make molded parts.
U.S. Pat. No. 4,749,617 discloses a composite article wherein two rigid materials are joined to one another via an intermediate layer made up of an aziridine-functional material and an organic resin.
EPO 86.304526.6 discloses the application of a composition consisting essentially of at least one aziridine functionality or group to a substrate. Subsequently applied layers of magnetic recording media, photographic layers, adhesives, radiation sensitive compositions and silanes can be applied to the aziridine-treated surface.